Top seed and former title holder John Isner downed Chung Hyeon 7-6 (7/5), 6-4 to book a semi-final clash with defending champion Jack Sock at the ATP claycourt tournament in Houston.
Isner, who lifted the trophy in 2013, saved all seven break points he against the rising South Korean star, firing 14 aces and winning 58 percent of his second serve points.

The US veteran was solid when it mattered most against the 19-year-old Chung, voted last year's Most Improved Player by his peers.

"That one game in the first set, I was down four break points," Isner said.

"Serving at 4-3 (in the second), I was down three break points and I had to hit a stab volley on one of them. That was huge. He played a good return game."

Although he pulled off the win, Isner said he expected to see Chung continue to rise.
"He's only going to get better, he's only going to get stronger, he's going to make all his shots better," Isner said.

"At 19 years old, he's got such an incredible ceiling right now."

Isner set up a clash with another young standout in Sock, who defeated seventh-seeded Cypriot Marcos Baghdatis 6-4, 6-4.

Sock, seeded fourth, converted two of his eight break point opportunities and that was enough to extend his Houston winning streak to seven matches.

Former champion Juan Monaco of Argentina ousted fifth-seeded American Sam Querrey 6-4, 6-4 to book a semi-final against third-seeded Spaniard Feliciano Lopez, a 6-2, 6-3 winner over American Tim Smyczek.

"Every match that I play here, especially on center court, I feel great, I feel confident," said Monaco, who won in Houston in 2012.

Monaco, now ranked 148th in the world, is in his first semi-final in more than a year. He has been slowed by injury in 2016, withdrawing from the Australian Open with a right wrist injury and retired from his first-round match in Rio de Janeiro with right shoulder trouble.

Monaco will face a tough task against Lopez, who leads their career head-to-head 4-2 and said after downing Smyczek that he was gaining confidence with every match.

"Sometimes for me (confidence) means a lot," Lopez said. "It's tough to play first rounds, normally, so once I start winning I can play better and better. And today, I played quite good."
Source: AFP